1. Field of the Invention
The present invention relates to a multi-layer ceramic substrate and a method of produce the multi-layer ceramic substrate. Specifically, it relates to a multi-layer ceramic substrate accommodating a passive component such as a capacitor and an inductor, and a method of producing the multi-layer ceramic substrate.
2. Description of the Related Art
In order to realize multi-functions, higher density and higher performance in a multi-layer circuit substrate such as a multi-layer ceramic substrate, it is effective to store sophisticated passive components in the multi-layer circuit substrate and apply a high density of wiring.
Multi-layer circuit substrates storing a passive component accordingly have been produced conventionally by various methods as follows:
First, there is a so-called thick film method where a dielectric paste is printed on green substrate sheets by thick film forming technology so that the green sheet can be laminated, pressed and baked for partially storing a capacitor inside the multi-layer ceramic substrate. However, the method is disadvantageous in the following points:
(1) Due to comparatively large irregularities in the paste film thickness and printing displacement, irregularities in the capacitor characteristics such as the capacity are comparatively large.
(2) Paste deformation generated in the pressing or baking step gives rise to irregularity in characteristics.
(3) Due to deterioration of the flatness in the printing step of the repetition of printing and lamination, it is difficult to increase the number of the laminations so that a large capacity can hardly be achieved in a capacitor.
Second, there is a method similar to the above-mentioned first method for producing a multi-layer circuit substrate with a resistor and a capacitor where a capacitor, resistor and the like are printed on the surface of a ceramic substrate with a thick film forming technology in multi-layers. However, similar to the above-mentioned first method, this method is disadvantageous in the following points:
(1) Characteristic irregularity due to the printing pattern displacement and the film thickness irregularity,
(2) Capacity limitation due to the limitation in the number of the lamination, and
(3) Flatness deterioration.
Third, there is a method of accommodating a dielectric member with a sheet form inside a multi-layer substrate as disclosed in Japanese Unexamined Patent Publication No. 59-17232. In this method, a dielectric sheet having the same area as the substrate is laminated interposed between substrate sheets, pressed, and baked. Accordingly, although problems including the characteristic irregularity such as capacity and limitation with respect to a larger capacity can be solved, the following problems come out:
(1) Due to the layer arrangement of the dielectric members inside the substrate, the device cannot be designed freely.
(2) Problems such as crosstalk of signals can be easily generated.
Fourth, there is a method of assembling a preliminarily sintered chip type ceramic passive component inside a laminated member comprising a plurality of ceramic green sheets as disclosed in Japanese Unexamined Patent Publication No. 61-288498. Although the problems in the above-mentioned first to third methods can be solved in this method, the contraction behavior in the X, Y, Z directions needs to be strictly controlled so that the materials which can be used as the ceramic is limited, and further the following problems are involved:
(1) Substrate flatness can be deteriorated easily.
(2) It is difficult to improve the size accuracy.
(3) It is difficult to provide minute wiring.
The methods for achieving high density wiring in a multi-layer circuit substrate include a method of pressing onto the upper and lower surfaces of a laminated member comprising a plurality of green sheets to be baked at a low temperature, green sheets which do not contract at the baking temperature of the laminated member, baking the same at a comparatively low temperature and eliminating the unsintered layer derived from the latter green sheets after baking (see Japanese Unexamined Patent Publication No. 4-243978 for example), and a method of further pressing the laminated member from above and below at the time of baking in the above-mentioned method (see Japanese Unexamined Patent Publication No. 5-503498 for example).
Since contraction can be hardly generated in the substrate surface direction, that is, in the X-Y direction in these methods, the size accuracy of the substrate obtained can be improved. Therefore, it is advantageous in that disconnection can hardly be generated even with high density wiring. However, these methods are not for providing a passive component inside the substrate.
As a fifth method for producing a multi-layer circuit substrate accommodating a passive component, Japanese Unexamined Patent Publication No. 9-92983 discloses a method combining the above-mentioned method of preventing the substrate contraction in the X-Y direction, and the method of partially accommodating a capacitor inside a multi-layer circuit substrate as a sheet or a thick film. This method is suitable for producing a multi-layer circuit substrate accommodating a passive component with high density wiring.
Since a dielectric member layer having the same area as the substrate is provided when the dielectric member part is formed with a sheet in the fifth method, the dielectric member layer is exposed at the substrate end face. Because the dielectric member layer needs to be dense in order to prevent moisture penetration, a sufficient density is achieved in the dielectric member layer by pressing the substrate from above and below at the time of baking. However, since the dielectric member layer shape is limited, problems are involved as in the above-mentioned third method using a dielectric member sheet in that:
(1) Due to the layer arrangement of the dielectric members inside the substrate, the device cannot be designed freely.
(2) Problems such as crosstalk of signals can be easily generated.
On the other hand, when the dielectric member part is formed with a thick film in the fifth method, a step of filling onto a concave part preliminarily provided in the substrate sheet corresponding to the area for forming the dielectric member part with a dielectric paste can be adopted. Although the problems of the thick film displacement and characteristics irregularity generated by the dielectric paste deformation problems in the thick film method mentioned above as the first method can be improved, the paste film thickness irregularity can be improved only to an insufficient degree. Furthermore, due to the difficulty of having the dielectric member part with a laminated structure, a problem of difficulty in obtaining a large capacity still remains.